Gas bearings for use with vacuum chambers and their application in lithographic projection apparatuses

ABSTRACT

A bearing for use in a vacuum chamber comprises a gas bearing discharging pressurised gas into a gap between two members to maintain a predetermined separation between those members. To avoid the gas forming the gas bearing being an unacceptable leak into the vacuum chamber, a vacuum pump is provided between the vacuum chamber and the gas bearing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to bearings for use in vacuum chambers.More particularly, the invention relates to the application of such adevice in lithographic projection apparatuses.

2. Discussion of Related Art

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The radiation system may alsoinclude elements operating according to any of these principles fordirecting, shaping or controlling the projection beam of radiation, andsuch elements may also be referred to below, collectively or singularly,as a “lens”. In addition, the first and second object tables may bereferred to as the “mask table” and the “substrate table”, respectively.Further, the lithographic apparatus may be of a type having two or moremask tables and/or two or more substrate tables. In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more stages while one ormore other stages are being used for exposures. Twin stage lithographicapparatuses are described in International Patent Applications WO98/28665 and WO 98/40791, for example.

Lithographic projection apparatuses can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target area (die)on a substrate (silicon wafer) which has been coated with a layer ofphotosensitive material (resist). In general, a single wafer willcontain a whole network of adjacent dies which are successivelyirradiated via the reticle, one at a time. In one type of lithographicprojection apparatus, each die is irradiated by exposing the entirereticle pattern onto the die in one go; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—which iscommonly referred to as a step-and-scan apparatus—each die is irradiatedby progressively scanning the reticle pattern under the projection beamin a given reference direction (the “scanning” direction) whilesynchronously scanning the wafer table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed v at which the wafertable is scanned will be a factor M times that at which the reticletable is scanned. More information with regard to lithographic devicesas here described can be gleaned from International Patent ApplicationWO 97/33205.

In a lithographic apparatus, the size of features that can be imagedonto the wafer is limited by the wavelength of the projection radiation.To produce integrated circuits with a higher density of devices andhence higher operating speeds, it is desirable to be able to imagesmaller features. Whilst most current lithographic projectionapparatuses employ ultraviolet light generated by mercury lamps orexcimer lasers, it has been proposed to use shorter wavelength radiationof around 13 nm. Such radiation is termed extreme ultraviolet (EUV) orsoft x-ray, and possible sources include laser plasma sources orsynchrotron radiation from electron storage rings. An outline design ofa lithographic projection apparatus using synchrotron radiation isdescribed in “Synchrotron radiation sources and condensers forprojection x-ray lithography”, J B Murphy et al, Applied Optics Vol. 32No. 24 pp 6920-6929 (1993).

Other proposed radiation types include electron beams and ion beams.These types of beam share with EUV the requirement that the beam path,including the mask, substrate and optical components, be kept in a highvacuum. This is to prevent absorption and/or scattering of the beam,whereby a total pressure of less than about 10⁻⁶ millibar is typicallynecessary for such charged particle beams. Wafers can be contaminated,and optical elements for EUV radiation can be spoiled, by the depositionof carbon layers on their surface, which imposes the additionalrequirement that hydrocarbon partial pressures should generally be keptbelow 10⁻⁸ or 10⁻⁹ millibar. Otherwise, for apparatuses using EUVradiation, the total vacuum need pressure only be 10⁻³ or 10⁻⁴ mbar,which would typically be considered a rough vacuum.

Further information with regard to the use of electron beams inlithography can be gleaned, for example, from U.S. Pat. No. 5,079,122and U.S. Pat. No. 5,260,151, as well as from EP-A-0 965 888.

Working in such a high vacuum imposes quite onerous conditions on thecomponents that must be put into the vacuum and on the vacuum chamberseals, especially those around any part of the apparatus where a motionmust be fed-through to components inside the chamber from the exterior.For components inside the chamber, materials that minimise or eliminatecontaminant and total outgassing, i.e. both outgassing from thematerials themselves and from gases adsorbed on their surfaces, shouldbe used. It would be very desirable to be able to reduce or circumventsuch restrictions.

Bearings in vacuum are a particular problem. Most lubricants areunsuitable for use in high vacuum conditions, particularly when lowhydrocarbon partial pressures are required. Unlubricated bearings areknown, but are subject to wear and cannot meet the speed of operationand lifetime requirements of lithography apparatuses. It is alsodifficult with conventional bearings to reduce the bearing gap belowabout 30 μm. Such a gap around a motion feed-through into the vacuumchamber would present an unacceptable leak.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved bearingthat can be used in a vacuum chamber of a lithographic projectionapparatus, e.g. to support a slidable plate sealing an aperture in thevacuum chamber, a member passing through an aperture in the vacuumchamber or a moveable object within the vacuum chamber, and can operateat high speed for a very great number of cycles.

According to the present invention, these and other objects are achievedin a lithographic projection apparatus that has:

a radiation system for supplying a projection beam of radiation;

a first object table provided with a mask holder for holding a mask;

a second object table provided with a substrate holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate, the lithographic projection apparatushas

a vacuum chamber having a wall enclosing at least one of the first andsecond object tables, the vacuum chamber wall having an aperturetherein;

a moveable sealing member for sealing the aperture;

a bearing for bearing the sealing member and maintaining a gap, betweenthe sealing member and the vacuum chamber wall, the bearing including

a gas bearing for providing pressurised gas into the gap thereby togenerate forces tending to hold the sealing member away from said vacuumchamber wall; and

an evacuation component spaced apart from the gas bearing for removingthe gas from the gap.

Another aspect of the invention provides a lithographic projectionapparatus that is:

a radiation system for supplying a projection beam of radiation;

a first object table provided with a mask holder for holding a mask;

a second object table provided with a substrate holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate. The lithographic projection apparatusalso has

a vacuum chamber having a wall enclosing at least one of the first andsecond object tables, the one object table being movable;

a bearing for displaceably bearing the one object table against abearing surface within the vacuum chamber and maintaining a gaptherebetween, the bearing including

a gas bearing for providing pressurised gas into the gap thereby togenerate forces tending to separate the borne and bearing members; and

an evacuation component spaced apart from the gas bearing for removinggas from the gap, the evacuating component being provided to surroundthe gas bearing.

Current lithography apparatuses are designed for use in clean roomenvironments and therefore some steps have conventionally been taken toreduce possible sources of contamination of wafers that are processed bythe apparatus. However, conventional designs of wafer, mask and transferstages are very complicated and employ large numbers of components forsensor and drive arrangements. Such stages also need to be provided withlarge numbers of signal and control cables and other utilities. Thepresent invention avoids the difficult and detailed task of making suchlarge numbers of components vacuum-compatible, or replacing them withvacuum-compatible equivalents, by adopting the principle of locating asmany components and functions as possible outside the vacuum chamber.The present invention thus avoids the need to vacuum-proof many or mostof the numerous components, by providing appropriate mechanicalfeed-throughs with innovative sealing arrangements. Likewise, thepresent invention avoids difficulties in reducing vibrations inevitablein vacuum apparatuses, particularly where powerful pumps are provided,by isolating as far as possible vibration sensitive components from thevacuum chamber wall.

The gas bearing of the invention maintains the desired separationbetween the borne and bearing members (e.g. the sealing member and thevacuum chamber wall or the object table and bearing surface in thevacuum chamber) , whilst the evacuation component prevents the gasforming the gas bearing being an unacceptable leak into the vacuumchamber. The vacuum bearing of the invention has many applications, forexample supporting a sliding seal plate, supporting a rod of a motionfeed-through into the vacuum chamber, supporting, an object or objecttable that must move on the vacuum chamber floor, or allowing a supportpillar to be isolated from the vacuum chamber wall.

The evacuation component may comprise an elongate groove located in onesurface defining the gap and connected by spaced-apart vacuum conduitsto a vacuum pump. Alternatively, multiple parallel grooves may beprovided and connected to separate vacuum pumps, with those groovesnearer the vacuum chamber drawing a deeper vacuum.

According to a further aspect of the invention there is provided amethod of manufacturing a device using a lithographic projectionapparatus that has:

a radiation system for supplying a projection beam of radiation;

a first object table provided with a mask holder for holding a mask;

a second object table provided with a substrate holder for holding asubstrate; and

a projection system for imaging an irradiated portion of the mask onto atarget portion of the substrate. The lithographic projection apparatusalso has

a vacuum chamber having a wall enclosing at least one of the first andsecond object tables, the vacuum chamber wall having an aperturetherein;

a moveable sealing member for sealing the aperture;

a bearing for bearing the sealing member and maintaining a gap betweenthe sealing member and the vacuum chamber wall, the bearing comprising:

a gas bearing for providing pressurised gas into the gap thereby togenerate forces tending to hold the sealing member away from the vacuumchamber wall; and

evacuation means spaced apart from the gas bearing for removing the gasfrom the gap; the method includes

mounting a mask on said first object table;

mounting a substrate on the second object table; and

exposing the substrate to an image of the mask.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping) metallisation,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN007-067250-4.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetarea”, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention and its attendant advantages will be describedbelow with reference to exemplary embodiments and the accompanyingschematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a cross-sectional view of a differential gas bearing accordingto a second embodiment of the invention;

FIG. 3 is a plan view of a variation of the differential gas bearing ofFIG. 2;

FIG. 3A is an enlarged cross-section of part of the differential gasbearing of FIG. 3;

FIG. 4 is a cross-sectional view of a wafer stage of a lithographicapparatus according to a third embodiment of the present invention;

FIG. 5 is a cross-section of an isolation mount according to a fourthembodiment of the invention; and

FIG. 6 is a cross-section of a wafer stage according to a fifthembodiment of the present invention.

In the various drawings, like parts are indicated by like references.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatus 1according to the invention. The apparatus comprises:

a radiation system LA, IL for supplying a projection beam PB ofradiation (e.g. UV or EUV radiation, electrons or ions);

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to a first positioningcomponent PM for accurately positioning the mask with respect to itemPL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning component means PW for accuratelypositioning the substrate with respect to item PL;

a projection system (“lens”) PL (e.g. a refractive or catadioptricsystem, a mirror group or an array of field deflectors) for imaging anirradiated portion of the mask MA onto a target portion C (die) of thesubstrate W.

The radiation system comprises a source LA (e.g. an undulator or wigglerprovided around the path of an electron beam in a storage ring orsynchrotron, a plasma source or an electron or ion beam source) whichproduces a beam of radiation. This beam is passed along various opticalcomponents included in illumination system IL, e.g. for the purpose ofshaping and/or collimating the resultant beam PB, and/or making ituniformly intense throughout its cross-section.

The beam PB subsequently impinges upon the mask MA which is held in amask holder on a mask table MT. Having been selectively reflected (ortransmitted) by the mask MA, the beam PB passes through the “lens” PL,which focuses the beam PB onto a target area C of the substrate W. Withthe aid of the positioning component PW and the interferometricdisplacement measuring device IF, the substrate table WT can be movedaccurately, e.g. so as to position different target areas C in the pathof the beam PB. Similarly, the positioning means PM and theinterferometric displacement measuring device IF can be used toaccurately position the mask MA with respect to the path of the beam PB,e.g. after mechanical retrieval of the mask MA from a mask library orduring a scanning motion. In general, movement of the object tables MT,WT will be realized with the aid of a long stroke module (coursepositioning) and a short stroke module (fine positioning), which are notexplicitly depicted in FIG. 1.

The depicted apparatus can be used in two different modes:

In step mode, the mask table MT is kept essentially stationary, and anentire mask image is projected in one go (i.e. a single “flash”) onto atarget area C. The substrate table WT is then shifted in the X and/or Ydirections so that a different target area C can be irradiated by thebeam PB;

In scan mode, essentially the same scenario applies, except that a giventarget area C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g. the X direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (e.g., M=¼ or⅕). In this manner, a relatively large target area C can be exposed,without having to compromise on resolution.

Embodiment 2

A gas bearing 21 (“air bearing”) according to a second embodiment of theinvention is generally depicted in FIG. 2. FIG. 2 is a cross-sectionthrough a differential gas-bearing 21, showing part of a supportingmember, e.g. vacuum chamber wall 11, and a supported member, e.g. asliding seal plate 12. Gas bearing 21 holds the plate 12 off the vacuumchamber wall by a constant gap, g, of 5 μm, for example. For such a gap,the surface 11 b of the vacuum chamber wall in the vicinity of thebearing, and the surface 12 a of the supported member over the area oftravel of the bearing, must be finished to an RMS surface roughness ofless than 0.8 μm, though they need not be flatter than 0.4 μm RMSsurface roughness. This can readily be achieved with known mechanicalpolishing techniques. In some applications a gap in the range of from 5μm to 10 μm may be appropriate and the surfaces need not be finished tosuch high tolerances. Clean air (or other gas, e.g. N₂) is suppliedcontinually through gas feed 211 at a pressure of several atmospheres togenerate a high pressure region 214. The supplied air will flow towardsa compartment M and also the vacuum chamber V, where its presence would,of course, be undesirable. An escape path to atmospheric pressure isprovided via groove 212. To prevent further the air that forms the airbearing becoming an unacceptable leak into the vacuum chamber V, it ispumped away via vacuum conduit 213. If desired, the escape path 212 mayalso be pumped. In this way, the residual leakage, 1, into the vacuumchamber V can be kept within acceptable levels.

In this embodiment the lower parts of the air feed 211 and the vacuumconduit 213 as well as the escape path 212 are elongate groovesextending along the entire length of the perimeter of the seal. Air feedpipes 211 a and vacuum pipes 213 a are provided at intervals along thegrooves.

In a variation to the second embodiment, shown in FIG. 3, which is aview from below of a part of the vacuum wall 11, the gas feeds 211′ thatprovide the gas bearing are discrete. At the end of each gas feed pipe211 a there is an enlargement 211 b, shown in cross-section in FIG. 3A,that is filled with a porous plug 211 c. The enlargement may becylindrical or any other convenient shape and may be omitted if desired.The porous plug 211 c is preferably made of graphite, which enables itto be placed in the enlargement 211 b after machine finishing of thelower surface 11 b of the vacuum wall 11, and then scraped smooth.

In both variants of the gas bearing described above, a single vacuumgroove 213 is provided between the air feed 211 and the vacuum chamberV. In other variants, two or more vacuum grooves may be provided, withthose nearer the vacuum chamber V being pumped to higher vacuum levels.

Embodiment 3

Part of a lithographic projection apparatus 2 according to a thirdembodiment of the invention is shown schematically and in cross-sectionin FIG. 4. This embodiment of the invention additionally makes use of asliding seal concept further described in European Patent ApplicationNo. 99201220.3 entitled “Motion Feed-Through into a Vacuum Chamber andits Application in Lithographic Apparatus” and a concurrently filed U.S.application of similar title (Applicant's ref: P-0130-010) which areincorporated herein by reference. The vacuum chamber V is bounded bywalls 11 which define an aperture 11 a in the floor of the chamber.During use of the apparatus the vacuum chamber V is kept at a sufficientvacuum by vacuum pumps (not shown) of appropriate type. The aperture 11a is sealed by a sliding seal formed by sliding seal plate 12 in themiddle of which is provided wafer support pillar 13. Pillar 13 supportsthe fine stage, or short stroke wafer support chuck, 14 which in turncarries the wafer W.

The longstroke motion of the wafer W, by which different areas of it arepositioned under the lens (not shown) of the lithographic apparatus forexposure, is accomplished by moving the whole sliding seal plate 12. Tothis end, the aperture 11 a is shaped and dimensioned to accommodate thedesired range of movement of the longstroke stage and the pillar 13. Inan apparatus intended to expose wafers of 300 mm diameter and with apillar of 100-150 mm diameter, for example, the aperture 11 a might be asquare of 480 mm sides to provide room for sensors, etc. around the edgeof the wafer. The sliding seal plate 12 must be of a shape and size tomaintain a seal over the aperture throughout its entire range ofmovement, and is in this example therefore also a square of 1200 mmsides, for example. This size allows a seal width of 120 mm each side. Acircular aperture and sealing plate may also be suitable.

Sliding seal plate 12 is driven to translate in orthogonal X- and Y-axesas well as to provide rotation, φ_(z), about the Z-axis, via beams 15and drivers 16 provided in motor compartment M.

It will be appreciated that, in use, the major load on the sliding sealplate 12 will be the pressure differential between the vacuum chamber Vand the motor compartment M, which is normally kept at atmosphericpressure (or a slightly different pressure in a clean room environment).This upward force (inward) will normally substantially exceed thegravitational force of the weight of the longstroke stage and the othercomponents it carries. Differentially pumped air bearings 21 accordingto the invention, which are preloaded by the difference between thepressure force and the weight of the sealing plate and the components itsupports, are provided around the aperture 11 a. To support the slidingseal plate 12 when the vacuum chamber is not evacuated, e.g. formaintenance, supports or bearings 19 mounted on base plate 17 areprovided.

Cables 20 providing control and measuring signals, as well as other“utilities”, to the shortstroke stage 14 are provided through a hole 12a in the sliding seal plate 12 and the hollow interior of the pillar 13.

To provide an adequate seal around the entire periphery of the aperture11 a, it is necessary to ensure that deformation of the sliding sealplate 12 is kept within acceptable limits. According to this embodimentof the invention this is effected by providing a plate of sintered Al₂O₃(e=3700 kg/m³, E=3.5×10¹¹ N/m², υ (Poisson's ratio)=0.22) of thickness100 mm. Other suitable materials, such as SiC foam, may also be used.

Embodiment 4

An isolation support 40 according to a fourth embodiment of theinvention is shown in FIG. 5. Isolation support 40 is designed to allowan object (not shown) in the vacuum chamber V to be supported whilstbeing isolated from the vacuum chamber wall 11. The object may be, forexample, the metrology frame of a lithography apparatus which isdesirably isolated from the vibrations in the vacuum chamber wall 11.The vacuum chamber wall is relatively prone to vibrations derived fromthe vacuum pumps, etc. which are attached to it.

The object to be supported is mounted on support pillar 41 (which may beone of a plurality of such pillars) which passes through an aperture 11a defined in the vacuum chamber wall 11. The aperture 11 a is sealed bya three-part seal 42 which allows the support 41 to move relative tovacuum chamber wall 11 in six degrees of freedom. The three-part seal 42comprises annular collar 43, seat 44 and plate 45. Collar 43 is providedaround the pillar 41 and has a convex hemispherical upper surface 43 a.The hemispherical surface 43 a sits in a complementary concavehemispherical surface 44 a in seat 44. The flat upper surface 44 b ofthe seat lies against plate 45.

Three differential gas bearings 21 a, b, c are provided in theinterfaces in the three-part seal; one in the collar 43 around thepillar 41, one in the seat 44 around the concave hemispherical surface44 a and one in the seat 44 around the flat upper surface 44 b. Bearing21 a allows the pillar 41 to be displaced relative to the seallongitudinally and also to rotate about its axis, providing freedom forthe pillar to move parallel to the Z-axis and also allowing φ_(z)rotation. Bearing 21 b allows the hemispherical collar 43 to rotateabout three axes, providing φ_(x) and φ_(y) rotational freedom, as wellas further go freedom. Bearing 21 c allows the seat 44 to move sidewaysrelative to plate 45, providing X and Y displacement freedom for thepillar 41.

Each of the bearings 21 a, b, c is a differential gas bearing accordingto the invention and comprises gas feeds 211 and evacuation component213.

The plate 45 is sealed to the vacuum chamber wall 11 by bolts 45 a and,e.g., O-ring 45 b in a conventional manner. In use, the pressuredifferential between the atmosphere and the vacuum chamber V will keepthe rest of the seal together, but additional constraints can beprovided for when the vacuum is released, e.g. for maintenance.

As described above, the three gas bearings between them allow sixdegrees of freedom to the pillar 41. The range of movement allowed ineach degree will depend, except in the case of φ_(z), on the diametersof the pillar, d, and of the aperture in plate 45, D. In thisembodiment, which aims to isolate support pillar 41 from vibrations inthe vacuum chamber wall, only a relatively small range of movement isnecessary.

Embodiment 5

A part of a lithographic apparatus according to a fifth embodiment ofthe invention is shown in cross-section in FIG. 6. In this embodiment anobject table T, for example the wafer table, is wholly contained withina vacuum chamber V and is moveable about on the floor 110 of the vacuumchamber V by a positioning component (not shown). To enable the movementof the object table T it is supported above the floor 110 of the vacuumchamber V by gas bearing 21 which surrounds the entire periphery of theobject table T.

The greater part of the gas provided to high pressure area 214 to formthe bearing flows inwardly to expansion chamber E from whence it isdrawn by pump P1 and recirculated to the gas supply 211. Some gas will,of course, flow outwardly from high pressure area 214 and the greaterpart of this flow is interrupted by groove 212 and led back to theexpansion chamber E by conduits 212 a. Such gas as continues to flowoutwardly is drawn into evacuation groove 213 by vacuum pump P2 and inthis way the residual gas flow 1 into the vacuum chamber V is keptwithin acceptable limits.

In FIG. 6 the pumps P1 and P2 are shown schematically outside the objecttable T. They may however be provided in the object table T, to make thetable self contained. In that case, a top-up gas supply may be providedin the table also.

The invention is described above in relation to preferred embodiments;however it will be appreciated that the invention is not limited by theabove description. In particular, the invention has been described abovein relation to the wafer stage of a lithographic apparatus but isequally applicable to the mask stage of such an apparatus or to anyapparatus in which a bearing must be provided in a vacuum chamber.

We claim:
 1. A lithographic projection apparatus, comprising: aradiation system constructed to supply a projection beam of radiation; afirst object table provided with a mask holder to hold a mask; a secondobject table provided with a substrate holder to hold a substrate; aprojection system constructed and arranged to image an irradiatedportion of the mask onto a target portion of the substrate; a vacuumchamber having a wall enclosing at least one of said first and secondobject tables, said vacuum chamber wall having an aperture therein; amoveable sealing member constructed and arranged to seal said aperture;a bearing constructed and arranged to bear said sealing member andmaintain a gap between said sealing member and said vacuum chamber wall,said bearing comprising: a gas bearing adapted to provide pressurizedgas into said gap thereby to generate forces tending to hold saidsealing member away from said vacuum chamber wall; and an evacuationsystem spaced apart from said gas bearing constructed and arranged toremove said gas from said gap.
 2. An apparatus according to claim 1,wherein said sealing member comprises a sealing plate moveable parallelto said vacuum chamber wall; and said gas bearing and evacuation systemare provided in said vacuum chamber wall to surround said aperture.
 3. Alithographic projection apparatus, comprising: a radiation systemconstructed to supply a projection beam of radiation; a first objecttable provided with a mask holder to hold a mask; a second object tableprovided with a substrate holder to hold a substrate; a projectionsystem constructed and arranged to image an irradiated portion of themask onto a target portion of the substrate; a vacuum chamber having awall enclosing at least one of said first and second object tables, saidat least one object table being movable; and a bearing adapted todisplaceably bear said at least one object table against a bearingsurface within said vacuum chamber and maintaining a gap therebetween,the bearing comprising: a gas bearing adapted to provide pressurised gasinto said gap thereby to generate forces tending to separate said atleast one object table from said bearing surface; and an evacuationsystem spaced apart from said gas bearing constructed and arranged toremove gas from said gap, said evacuating system being provided tosurround said gas bearing.
 4. An apparatus according to claim 1, furthercomprising a conduit providing a fluid communication between said gapand a reservoir at a pressure higher than that of said vacuum chamberand lower than that of said pressurized gas.
 5. An apparatus accordingto claim 4, wherein said conduit comprises an elongate groove in onesurface defining said gap.
 6. An apparatus according to claim 1, whereina distance along said gap between a point of supply of gas to form saidgas bearing and said evacuation system is greater than a distancebetween said evacuation system and an adjacent vacuum chamber.
 7. Anapparatus according to claim 1, wherein said evacuation system comprisesan elongate vacuum groove in one surface defining said gap; a vacuumpump; and vacuum conduits connecting said elongate vacuum groove to saidvacuum pump.
 8. An apparatus according to claim 7, wherein saidevacuation system further comprises at least one second elongate vacuumgroove generally parallel to said elongate vacuum groove and provided onthe other side thereof than said gas bearing, a second vacuum pump andvacuum conduits connecting said second elongate vacuum groove to saidsecond vacuum pump, said second vacuum pump drawing a deeper vacuum thansaid vacuum pump.
 9. An apparatus according to claim 1, wherein said gasbearing comprises an elongate groove in one surface defining said gap;and gas supply conduits structured to supply gas under pressure to saidelongate groove.
 10. An apparatus according to claim 1, wherein said gasbearing comprises a plurality of spaced apart indentations in onesurface defining said gap, said indentations being filled with porousmaterial; and gas supply conduits structured to supply gas underpressure to said indentations.
 11. A method of manufacturing a deviceusing a lithographic projection apparatus, comprising: supplying aprojection beam of radiation; holding a mask with a mask holder of afirst object table; holding a substrate with a substrate holder of asecond object table; imaging an irradiated portion of the mask onto atarget portion of the substrate; sealing an aperture defined by a wallof a vacuum chamber of said lithographic projection apparatus with amovable sealing member, said wall of said vacuum chamber enclosing atleast one of said first and second object tables; providing pressurizedgas into a gap between said sealing member and said vacuum chamber wallto generate forces tending to hold said sealing member away from saidvacuum chamber wall; removing said gas from said gap; and exposing saidsubstrate to an image of said mask.
 12. A device manufactured accordingto the method of claim
 11. 13. An apparatus according to claim 3,further comprising a conduit providing a fluid communication betweensaid gap and a reservoir at a pressure higher than that of said vacuumchamber and lower than that of said pressurized gas.
 14. An apparatusaccording to claim 13, wherein said conduit comprises an elongate groovein one surface defining said gap.
 15. An apparatus according to claim 3,wherein a distance along said gap between a point of supply of gas toform said gas bearing and said evacuation system is greater than adistance between said evacuation system and an adjacent vacuum chamber.16. An apparatus according to claim 3, wherein said evacuation systemcomprises an elongate vacuum groove in one surface defining said gap; avacuum pump; and vacuum conduits connecting said elongate vacuum grooveto said vacuum pump.
 17. An apparatus according to claim 16, whereinsaid evacuation system further comprises at least one second elongatevacuum groove generally parallel to said elongate vacuum groove andprovided on the other side thereof than said gas bearing, a secondvacuum pump and vacuum conduits connecting said second elongate vacuumgroove to said second vacuum pump, said second vacuum pump drawing adeeper vacuum than said vacuum pump.
 18. An apparatus according to claim3, wherein said gas bearing comprises an elongate groove in one surfacedefining said gap; and gas supply conduits structured to supply gasunder pressure to said elongate groove.
 19. An apparatus according toclaim 3, wherein said gas bearing comprises a plurality of spaced apartindentations in one surface defining said gap, said indentations beingfilled with porous material; and gas supply conduits structured tosupply gas under pressure to said indentations.
 20. An apparatusaccording to claim 1, further comprising a support pillar passingthrough the aperture; wherein said sealing member comprises a seatdisplaceable relative to said vacuum chamber wall in at least one degreeof freedom and a collar displaceable relative to said seat in at leastone degree of freedom, said collar supporting said support pillar;wherein said bearing is adapted to bear said seat relative to saidvacuum chamber wall by providing pressurized gas to a gap between saidseat and said vacuum chamber wall and to remove gas from the gap betweensaid seat and said vacuum chamber; and wherein said bearing is adaptedto bear said collar relative to said seat by providing pressurized gasto a gap between said collar and said seat and to remove gas from thegap between said collar and said seat.
 21. An apparatus according toclaim 20, wherein said bearing is adapted to bear said support pillarrelative to said collar by providing pressurized gas to a gap betweensaid collar and said support pillar and to remove gas from the gapbetween said collar and said support pillar.
 22. An apparatus accordingto claim 21, wherein said seat is displaceable relative to said vacuumchamber wall in at least two degrees of freedom, said collar isdisplaceable relative to said seat in at least two degrees of freedomand said support pillar is displaceable relative to said collar in atleast two degrees of freedom.
 23. A method according to claim 11,comprising removing said gas from said gap using a pressure higher thanthat of said vacuum chamber and lower than that of said pressurised gas.24. A method of manufacturing a device using a lithographic projectionapparatus, comprising: supplying a projection beam of radiation; holdinga mask with a mask holder of a first object table; holding a substratewith a substrate holder of a second object table; imaging an irradiatedportion of the mask onto a target portion of the substrate; displaceablybearing at least one of said first and second object tables, in a vacuumchamber, against a bearing surface; providing pressurised gas into a gapbetween said bearing surface and said at least one object table togenerate forces tending to separate said at least one object table fromsaid bearing surface and maintaining the gap therebetween; removing gasfrom said gap; and exposing said substrate to an image of said mask. 25.A method according to claim 24, comprising removing said gas from saidgap using a pressure higher than that of said vacuum chamber and lowerthan that of said pressurised gas.